Method for preparing elemental sulfur as a diffusion-resistant gas and methods for its use in making lime, sulfur dioxide and sulfuric acid from waste gypsum

Abstract

The present invention relates to the discovery of a method by which elemental sulfur may be delivered to a reaction zone, for gas-solids reaction, in the form of a concentrated, coherent, diffusion-resistant gas thereby permitting gas-solid reactions of greatly enhanced reaction rates. It has been found that sulfur, which is first vaporized at approximately 1 atmosphere pressure, and then super heated to at least 1270° F. while in transit through a confined space, such as a transfer line, will debouch from said confined space into a reaction zone as a coherent gas which resist diffusion throughout the free space of the reaction zone. This coherent form of sulfur gas exhibits a dark reddish-violet color. The coherent form of sulfur gas, since it resists diffusion throughout the free space of the reaction zone, flows into concentrated contact with, and remains concentrated at, the solids surface, thereby producing a gas-solids reaction of greatly enhanced reaction rate. Utilizing the above discovery a process has been devised by which the sulfur values present in gypsum, particularly waste by-product gypsum produced by the wet process, may be recovered as SO 2 with the concurrent production of lime.

I claim: 
     
       1. A method for preparing elemental sulfur as a coherent diffusion resistant gas for reaction with solid reactants, comprising the steps of: heating elemental sulfur to the boiling point of sulfur in an enclosed vessel which communicates with a delivery conduit to continuously supply sulfur vapor to said conduit;   super heating said sulfur vapor during transit through said delivery conduit to a temperature of at least about 1270° F.; and   debouching said super heated sulfur vapor from said conduit into a reaction zone to which solid reactant is continuously supplied, whereby said super heated sulfur vapor debouches in the form of a coherent diffusion resistant gas which flows into concentrated contact with said solid reactant.   
     
     
       2. A method for obtaining sulfur dioxide by reacting calcium sulfate with a coherent diffusion resistant gaseous reagent form of elemental sulfur, comprising the steps of: continuously supplying calcium sulfate to a reaction zone maintained at a temperature of from about 1600° F. to about 2300° F.;   heating elemental sulfur to the boiling point of sulfur in an enclosed vessel which communicates with a delivery conduit to continuously supply sulfur vapor to said conduit;   super heating said sulfur vapor during transit through said delivery conduit to a temperature of at least about 1270° F.; and   debouching said super heated sulfur vapor from said conduit into said reaction zone as a coherent gas which resists diffusion within said reaction zone and thus flows into concentrated contact with said calcium sulfate whereby calcium sulfate is converted by reaction with said sulfur into calcium sulfide and sulfur dioxide.   
     
     
       3. The method of claim 1 or 2, wherein said sulfur vapor is heated during transit through said delivery conduit to a temperature of from about 1300° F. to about 1800° F. 
     
     
       4. A method for obtaining sulfur dioxide and lime from calcium sulfate, comprising the steps of: continuously supplying calcium sulfate to a first reaction zone maintained at a temperature of from about 1600° F. to about 2300° F.;   heating elemental sulfur to the boiling point of sulfur in an enclosed vessel which communicates with a delivery conduit to continuously supply sulfur vapor to said conduit;   super heating said sulfur vapor during transit through said delivery conduit to a temperature of at least about 1270° F.;   debouching said super heated sulfur vapor from said conduit into said first reaction zone as a coherent gas which resists diffusion in said first zone and thus flows into concentrated contact with said calcium sulfate whereby a portion of said calcium sulfate is converted into calcium sulfide and sulfur dioxide; and   supplying said calcium sulfide and said unreacted portion of calcium sulfate to a second reaction zone which is held at a temperature of at least about 1800° F. and maintaining said solids in said second zone for a time sufficient to permit essentially complete reaction between calcium sulfate and calcium sulfide to produce lime and sulfur dioxide.   
     
     
       5. The method of claim 4 wherein said sulfur vapor during transit through said delivery conduit is heated to a temperature of from about 1300° F. to about 1800° F. 
     
     
       6. The method of claim 5 wherein the temperature of said second reaction zone is held at from about 1950° F. to about 2300° F. 
     
     
       7. A method for recovering sulfur values from waste by-product gypsum produced by the wet process production of phosphoric acid as sulfur dioxide, comprising the steps of: dehydrating gypsum to convert gypsum to calcium sulfate;   supplying said calcium sulfate to a first reaction zone maintained at a temperature of from about 1600° F. to about 2300° F.;   heating elemental sulfur to the boiling point of sulfur in an enclosed vessel which communicates with a delivery conduit to continuously supply sulfur vapor to said conduit;   super heating said sulfur vapor during transit through said delivery conduit to a temperature of from about 1300° F. to about 1800° F.;   debouching said super heated sulfur vapor from said conduit into said first reaction zone as a coherent gas which resists diffusion within said first zone and thus flows into concentrated contact with said calcium sulfate whereby a portion of said calcium sulfate is converted into calcium sulfide and sulfur dioxide; and   supplying said calcium sulfide and said unreacted portion of calcium sulfate to a second reaction zone which is held at a temperature of from about 1800° F. to about 2400° F. and maintaining said solids in said second zone for a time sufficient to permit essentially complete reaction between calcium sulfide and calcium sulfate to produce lime and sulfur dioxide.   
     
     
       8. A method for recovering sulfur values from waste by-product gypsum produced by the wet process production of phosphoric acid and recycling such sulfur values as sulfuric acid for utilization as a reagent in the wet process, comprising the steps of: dehydrating said gypsum to convert gypsum to calcium sulfate;   supplying said calcium sulfate to a first reaction zone maintained at a temperature of from about 1600° F. to about 2300° F.;   heating elemental sulfur to the boiling point of sulfur in an enclosed vessel which communicates with a delivery conduit to continuously supply sulfur vapor to said conduit;   super heating said sulfur vapor during transit through said delivery conduit to a temperature of from about 1300° F. to about 1800° F.;   debouching said super heated sulfur vapor from said conduit into said first reaction zone as a coherent gas which resists diffusion within said first zone and thus flows into concentrated contact with said calcium sulfate whereby a portion of said calcium sulfate is converted into calcium sulfide and sulfur dioxide;   supplying said calcium sulfide and said unreacted portion of calcium sulfate to a second reaction zone which is held at a temperature of from about 1950° F. to about 2400° F. and maintaining said solids in said second zone for a time sufficient to permit essentially complete reaction between calcium sulfide and calcium sulfate to produce lime and sulfur dioxide;   passing a non-reacting gas in counter-current flow through said second and first zone to collect and remove the sulfur dioxide generated therein from said zones; and   converting said collected sulfur dioxide to sulfuric acid.